1. Field of the Invention
The present invention relates to a laminated balun transformer, and more particularly, to a laminated balun transformer subminiaturized with a transmission line length reduced below λ/4 without any variation of characteristics.
2. Description of the Related Art
A term “balun” in a balun transformer, which is the abbreviation for balance to unbalance, refers to a device composed of a circuit or structure for transforming a balanced signal to an unbalanced signal and vice versa. For example, this is required to perform a transformation between the balanced signal and the unbalanced signal when a device having a balanced input/output stage, such as a mixer and an amplifier, is connected to a device having an unbalanced input/output stage, such as an antenna.
The balun transformer can be implemented by a combination of lumped elements such as R, L, and C elements, or distributed elements such as a microstrip line, a strip line and a transmission line. Recently, with an increasing need for miniaturization of radio communication products widely employing the balun transformer, a laminated balun transformer using low temperature cofired ceramics (LTCC) is being frequently used to reduce the size of the elements.
FIG. 1 is an equivalent circuit showing a basic configuration of a general balun transformer suggested by Marchand. As shown in this figure, the balun transformer is composed of four strip lines 14 to 17 each having a length of λ/4 (where, λ is 1/fc (fc is a center frequency of an input/output signal)). Of the four strip lines 14 and. 17, a first strip line 14 has one end connected to an unbalanced port 11 through which an unbalanced signal having a certain frequency is inputted or outputted. A second strip line 15 has one end connected in series to the other end of the first strip line 14. The other end of the second strip line 15 is opened. In addition, a third strip line 16 and a fourth strip line 17 have respective one ends connected to a ground point and are arranged in parallel with the first strip line 14 and the second strip line 15, respectively, to form an electrical coupling between them. The other ends of the third and fourth strip lines 16 and 17 are connected respectively to balanced ports 12 and 13 through which a balance signal is inputted or outputted.
In the above configuration, the first strip line 14 and the third strip line 16 form an coupler and the second strip line 15 and the fourth strip line 17 form another coupler. With the above configuration, when the unbalanced signal having the certain frequency is inputted to the unbalanced port 11, an electromagnetic coupling among the first to fourth strip lines 14 to 17 is generated, and accordingly, the balanced signal having the same frequency and magnitude as the inputted unbalanced signal and but having a phase, which is different by 180 degrees from a phase of the unbalanced signal, is outputted through the balanced ports 12 and 13. On the contrary, when two balanced signals having different frequencies, the same magnitude, and phases, which are different by 180 degrees from each other, are respectively inputted to the balanced ports 12 and 13, an unbalanced signal having the same frequency as the two inputted balanced signals are outputted through the unbalanced port 11.
A conventional laminated balun transformer having such an equivalent circuit has a shape as shown in FIG. 2 and an internal structure as shown in FIG. 3.
Referring to FIG. 2, the conventional laminated balun transformer 20 is composed of a rectangular hexahedral dielectric block 21 and a plurality of external electrodes 23 to 28 formed on two opposite sides of the dielectric block 21, each of which is set as a terminal such as an unbalanced terminal, a balanced terminal, or a ground terminal. For example, an external electrode 3 is set as a terminal for non-connection, external electrodes 24 and 27 are set as a terminal for a ground, external electrodes 25 and 28 are set as a terminal for input/output of a balanced signal, and an external electrode 26 is set as a terminal for input/output of an unbalanced signal.
Referring to FIG. 3, the dielectric block 21 is composed of a plurality of dielectric sheets laminated using an LTCC method. On the plurality of dielectric sheets laminated are formed a first ground electrode 31a connected to the external electrodes 24 and 27 for a ground, the first strip line 14 having a length of λ/4 and having one end connected to the external electrode 26 for input/output of the unbalanced signal, the third strip line 16 formed in parallel with the first strip line 14, having a length of λ/4 and having both ends connected respectively to the external electrode 25 for input/output of the balanced signal and the external electrode 27 for a ground, a second ground electrode 31b connected to the external electrodes 24 and 27 for a ground, the second strip line 15 having a length of λ/4 and having one end connected to the first strip line 14 via the external electrode 23 and the other end opened, the fourth strip line 17 formed in parallel with the second strip line 15 and having both ends connected respectively to the external electrode 27 for a ground and the external electrode 28 for input/output of the balanced signal, and a third ground electrode 31c connected to the external electrodes 24 and 27 for a ground, sequentially in a downward direction.
Additionally, on the plurality of dielectric sheets laminated may be formed lead electrodes 32a to 32d for connecting the first to fourth strip lines 14 to 17 to respective external electrodes 23 to 28, and via holes 33a to 33d for electrically connecting the lead electrodes 32a to 32d to corresponding strip lines 14 to 17 on other layers.
As described above, the laminated balun transformer is miniaturized by vertically laminating four λ/4 strip lines. However, there is a limitation to the miniaturization of the laminated balun transformer due to an area required to implement the λ/4 strip lines.